It is estimated that in excess of 50 percent of adults in the United States are overweight (body mass index >25 kg/m2). Of particular concern is the significant increase in the prevalence of obesity (body mass index >30 kg/m 2) over the past few decades. Exercise appears to be an important behavior for preventing and treating a number of chronic diseases, and the current public health recommendation for exercise is 30 minutes of moderate intensity activity on most days of the week (150 minutes per week total). However, it is unclear whether this level of exercise is adequate for preventing weight gain and the development of obesity. Therefore, the purpose of this study is to examine the effect of two doses of exercise on prevention of weight gain in moderately overweight adults. Two-hundred ninety four (294) subjects will be recruited to participate in this study. Subjects will be 18-50 years of age, with a body mass index ranging from 25.0 to 29.9 kg/m 2. Subjects will be recruited in three cohorts, with 98 subjects per cohort. Subjects will be randomly assigned to one of the following three treatment groups: 1) control, 2) moderate exercise, or 3) high exercise. All subjects will be followed for a period of 18 months. The control group will receive basic health information in a newsletter at 3 month intervals. Subjects in the moderate exercise group will progress to 150 minutes of at least moderate intensity exercise. Subjects in the high exercise group will progress to approximately 300 minutes of at least moderate intensity exercise. Subjects in both the moderate and high exercise groups will attend weekly behavioral sessions weekly for 6 months, and biweekly thereafter along with 1 telephone contact per month. The primary outcome of this study will be change in body weight from baseline to 18 months. In addition, subjects will undergo assessment of body composition (using DEXA) and cardiorespiratory fitness at 0 and 18 months. We will also assess changes in physical activity, diet, and potential mediators of change in behaviors at 3-month intervals throughout this study. The results of this study will have a significant impact on exercise recommendations for preventing weight gain in adults.